KARYOKINESIS. 61 



plasm around the deeply staining body at the center of the aster. There is a faintly 

 staining zone surrounding the central body in the prophases both of the maturation 

 and cleavage divisions (see figs. 4-7, 26— 30, 52-56, 70-72, 76), but this zone accord- 

 ing to Boveri's definition, does not belong to the centrosome, since even at its first 

 appearance (cf. fig. 70) it is traversed by radiations; furthermore, a study of con- 

 secutive stages shows that it develops step by step into the inner portion of the 

 sphere. On the other hand, I believe that I have followed the central, deeply 

 staining body through every stage of its growth and metamorphosis, having seen it 

 not merely in the stages represented in the plates, but in thousands of others, many 

 of which were carefully drawn. The result of this study convinces me that the 

 small, deeply staining granule of the early prophase becomes the dense, spherical 

 body of the metaphase and the large, hollow sphere of the anaphase, and that this 

 body is the centrosome. The fact that in the mollusks generally the peripheral 

 layer of the centrosome stains more densely than the central portion, makes it 

 unusually easy in these animals to distinguish between the centrosome and the sur- 

 rounding sphere. The result therefore of the re-examination of my preparations in 

 the light of Boveri's work does not in any respect lessen my confidence in the accu- 

 racy of my observations and interpretations, at least as far as Crepidula is con- 

 cerned. 



The type of centrosome represented by Crepidula, Unio, Haminea and Aeolis, 

 viz., one within which the new centers and central spindles arise from the centriole 

 while a considerable part of the mother centrosome fades away into the sphere, 

 agrees much more "closely with the types of centrosomes found in Ascaris, Thalas- 

 semia and Echinus, than does that of Diaulula. Boveri represents these four types 

 in text figures (pp. 102-103), and it can be seen at a glance that in the first three 

 types the daughter centers and spindles occupy but a small part of the old centro- 

 some, whereas in the fourth type {Diaulula) they occupy the entire centrosome. I 

 have found that the relative size of the central spindle and daughter centrosomes 

 (Netrum of Boveri), as compared with the inclosing centrosome, differs considerably 

 in different cleavages of the egg. Thus in the first, second and third cleavages the 

 netrum is much smaller than the mother centrosome, whereas in the fourth, fifth 

 and later cleavages the netrum almost entirely fills the mother centrosome. In 

 view of these facts I venture to suggest that a re-examination of Diaulula with 

 regard to this point might show that the outlines of the netrum are not coincident 

 with those of the mother centrosome. but that the former lies within the latter as 

 is the case in the other mollusks named above, as well as in other types of centro- 

 somes described by Boveri. 



(<?) The Centrosome as a Persistent Cell Organ. — There is no more perplexing 

 problem in connection with the cell than that of the significance of the centrosome. 

 On the one hand there are the well established facts as to (1) its persistence from 

 cell cycle to cell cycle (my own observations showing that in the cleavage of Crep- 

 idula it persists without interruption to a stage with more than sixty cells and 

 probably throughout the entire development) ; (2) its independent growth and 



